nextTick 在 vue 2.5 和 vue 2.6 之间有什么不同

我们知道对于 Vue 来说，从数据变化到执行 DOM 更新，这个过程是 异步 的，发生在下一个 tick 里。

它会创建一个更新队列 queue ，队列中维护着各个属性的 watcher ，在需要时执行、更新它们。

在下次 DOM 更新循环结束之后执行延迟回调。在修改数据之后立即使用这个方法，获取更新后的 DOM。

Vue.nextTick()
    .then(function () {
    	// DOM 更新了
	})
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那么针对这样一个核心功能，Vue 2.5 与 Vue 2.6 的实现有什么不同呢？

可能需要你简单了解下 js 的 event loop。 模拟实现 JS 引擎：深入了解 JS机制 以及 Microtask and Macrotask

Vue 2.5 nextTick 实现

在 Vue 2.5 中，nextTick 的实现是 microTimerFunc 、 macroTimerFunc 组合实现的，具体见源码。

import { noop } from 'shared/util'
import { handleError } from './error'
import { isIOS, isNative } from './env'

const callbacks = []
let pending = false

function flushCallbacks () {
  pending = false
  const copies = callbacks.slice(0)
  callbacks.length = 0
  for (let i = 0; i < copies.length; i++) {
    copies[i]()
  }
}

// Here we have async deferring wrappers using both microtasks and (macro) tasks.
// In < 2.4 we used microtasks everywhere, but there are some scenarios where
// microtasks have too high a priority and fire in between supposedly
// sequential events (e.g. #4521, #6690) or even between bubbling of the same
// event (#6566). However, using (macro) tasks everywhere also has subtle problems
// when state is changed right before repaint (e.g. #6813, out-in transitions).
// Here we use microtask by default, but expose a way to force (macro) task when
// needed (e.g. in event handlers attached by v-on).
let microTimerFunc
let macroTimerFunc
let useMacroTask = false

// Determine (macro) task defer implementation.
// Technically setImmediate should be the ideal choice, but it's only available
// in IE. The only polyfill that consistently queues the callback after all DOM
// events triggered in the same loop is by using MessageChannel.
/* istanbul ignore if */
if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
  macroTimerFunc = () => {
    setImmediate(flushCallbacks)
  }
} else if (typeof MessageChannel !== 'undefined' && (
  isNative(MessageChannel) ||
  // PhantomJS
  MessageChannel.toString() === '[object MessageChannelConstructor]'
)) {
  const channel = new MessageChannel()
  const port = channel.port2
  channel.port1.onmessage = flushCallbacks
  macroTimerFunc = () => {
    port.postMessage(1)
  }
} else {
  /* istanbul ignore next */
  macroTimerFunc = () => {
    setTimeout(flushCallbacks, 0)
  }
}

// Determine microtask defer implementation.
/* istanbul ignore next, $flow-disable-line */
if (typeof Promise !== 'undefined' && isNative(Promise)) {
  const p = Promise.resolve()
  microTimerFunc = () => {
    p.then(flushCallbacks)
    // in problematic UIWebViews, Promise.then doesn't completely break, but
    // it can get stuck in a weird state where callbacks are pushed into the
    // microtask queue but the queue isn't being flushed, until the browser
    // needs to do some other work, e.g. handle a timer. Therefore we can
    // "force" the microtask queue to be flushed by adding an empty timer.
    if (isIOS) setTimeout(noop)
  }
} else {
  // fallback to macro
  microTimerFunc = macroTimerFunc
}

/**
 * Wrap a function so that if any code inside triggers state change,
 * the changes are queued using a (macro) task instead of a microtask.
 */
export function withMacroTask (fn: Function): Function {
  return fn._withTask || (fn._withTask = function () {
    useMacroTask = true
    const res = fn.apply(null, arguments)
    useMacroTask = false
    return res
  })
}

export function nextTick (cb?: Function, ctx?: Object) {
  let _resolve
  callbacks.push(() => {
    if (cb) {
      try {
        cb.call(ctx)
      } catch (e) {
        handleError(e, ctx, 'nextTick')
      }
    } else if (_resolve) {
      _resolve(ctx)
    }
  })
  if (!pending) {
    pending = true
    if (useMacroTask) {
      macroTimerFunc()
    } else {
      microTimerFunc()
    }
  }
  // $flow-disable-line
  if (!cb && typeof Promise !== 'undefined') {
    return new Promise(resolve => {
      _resolve = resolve
    })
  }
}
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比较关键的几行

let useMacroTask = false

if (!pending) {
    pending = true
    if (useMacroTask) {
    	macroTimerFunc()
    } else {
    	microTimerFunc()
    }
}
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这里说明，Vue 2.5 会优先使用 microTimerFunc ，如果存在兼容性问题，则降级为 macroTimerFunc 。

microTimerFunc 的实现：原生的 Promise ；

macroTimerFunc 的实现： setImmediate || MessageChannel || setTimeout 。

同时 Vue 2.5 的 next-tick 还对外暴露了两个函数： nextTick 以及 withMacroTask （用于处理一些 DOM 交互事件，如 v-on 绑定的事件回调函数的处理，会强制走 macro task）。

通读源码，发现逻辑很清晰，也完成优雅渐进，那么发什么了导致 Vue 在 2.6 对其进行了 fix 呢？来看一段 Vue 2.6 的注释

// Here we have async deferring wrappers using microtasks.
// In 2.5 we used (macro) tasks (in combination with microtasks).
// However, it has subtle problems when state is changed right before repaint
// (e.g. #6813, out-in transitions).
// Also, using (macro) tasks in event handler would cause some weird behaviors
// that cannot be circumvented (e.g. #7109, #7153, #7546, #7834, #8109).
// So we now use microtasks everywhere, again.
// A major drawback of this tradeoff is that there are some scenarios
// where microtasks have too high a priority and fire in between supposedly
// sequential events (e.g. #4521, #6690, which have workarounds)
// or even between bubbling of the same event (#6566).
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主要阐明了两个问题：

• 在重绘之前状态发生改变会有轻微的问题；
• 利用 macro task 处理事件时，会产生一系列无法规避的诡异问题。

简单地描述下这两个问题：

第一个问题，具体见图

试一试，具体描述见 github.com/vuejs/vue/i… ，本质上就是在 css 中定义了 @media 媒体查询，js 中 window 监听了 resize 事件，那么当触发固定阈值时，state 发生了变化、样式也需要重绘，这就产生了问题。

第二个问题，一般可以概括为由于使用 macroTask 处理 DOM 操作，会使得有些时候触发和执行之间间隔太大，例如在移动端，单击的 handler 和音频播放功能不在同一 tick 里。

Vue2.6 nextTick 实现

由于以上问题，所以在 Vue 2.6 实现过程中，利用 microtasks 代替之前的解决方案，具体见源码。

/* @flow */
/* globals MutationObserver */

import { noop } from 'shared/util'
import { handleError } from './error'
import { isIE, isIOS, isNative } from './env'

const callbacks = []
let pending = false

function flushCallbacks () {
  pending = false
  const copies = callbacks.slice(0)
  callbacks.length = 0
  for (let i = 0; i < copies.length; i++) {
    copies[i]()
  }
}

// Here we have async deferring wrappers using microtasks.
// In 2.5 we used (macro) tasks (in combination with microtasks).
// However, it has subtle problems when state is changed right before repaint
// (e.g. #6813, out-in transitions).
// Also, using (macro) tasks in event handler would cause some weird behaviors
// that cannot be circumvented (e.g. #7109, #7153, #7546, #7834, #8109).
// So we now use microtasks everywhere, again.
// A major drawback of this tradeoff is that there are some scenarios
// where microtasks have too high a priority and fire in between supposedly
// sequential events (e.g. #4521, #6690, which have workarounds)
// or even between bubbling of the same event (#6566).
let timerFunc

// The nextTick behavior leverages the microtask queue, which can be accessed
// via either native Promise.then or MutationObserver.
// MutationObserver has wider support, however it is seriously bugged in
// UIWebView in iOS >= 9.3.3 when triggered in touch event handlers. It
// completely stops working after triggering a few times... so, if native
// Promise is available, we will use it:
/* istanbul ignore next, $flow-disable-line */
if (typeof Promise !== 'undefined' && isNative(Promise)) {
  const p = Promise.resolve()
  timerFunc = () => {
    p.then(flushCallbacks)
    // In problematic UIWebViews, Promise.then doesn't completely break, but
    // it can get stuck in a weird state where callbacks are pushed into the
    // microtask queue but the queue isn't being flushed, until the browser
    // needs to do some other work, e.g. handle a timer. Therefore we can
    // "force" the microtask queue to be flushed by adding an empty timer.
    if (isIOS) setTimeout(noop)
  }
} else if (!isIE && typeof MutationObserver !== 'undefined' && (
  isNative(MutationObserver) ||
  // PhantomJS and iOS 7.x
  MutationObserver.toString() === '[object MutationObserverConstructor]'
)) {
  // Use MutationObserver where native Promise is not available,
  // e.g. PhantomJS, iOS7, Android 4.4
  // (#6466 MutationObserver is unreliable in IE11)
  let counter = 1
  const observer = new MutationObserver(flushCallbacks)
  const textNode = document.createTextNode(String(counter))
  observer.observe(textNode, {
    characterData: true
  })
  timerFunc = () => {
    counter = (counter + 1) % 2
    textNode.data = String(counter)
  }
} else if (typeof setImmediate !== 'undefined' && isNative(setImmediate)) {
  // Fallback to setImmediate.
  // Techinically it leverages the (macro) task queue,
  // but it is still a better choice than setTimeout.
  timerFunc = () => {
    setImmediate(flushCallbacks)
  }
} else {
  // Fallback to setTimeout.
  timerFunc = () => {
    setTimeout(flushCallbacks, 0)
  }
}

export function nextTick (cb?: Function, ctx?: Object) {
  let _resolve
  callbacks.push(() => {
    if (cb) {
      try {
        cb.call(ctx)
      } catch (e) {
        handleError(e, ctx, 'nextTick')
      }
    } else if (_resolve) {
      _resolve(ctx)
    }
  })
  if (!pending) {
    pending = true
    timerFunc()
  }
  // $flow-disable-line
  if (!cb && typeof Promise !== 'undefined') {
    return new Promise(resolve => {
      _resolve = resolve
    })
  }
}
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Vue 2.6 利用最典型的两个 microTask， promise.then 以及 Mutation observers ，并添加 setImmediate 、 setImmediate ，作为降级方案。

只对外暴露了一个接口 next-tick，同时用 microTask 来处理 event handler。这种实现方法解决了上述问题，但是也有一个很明显的弊端。由于 microTask 的优先级太高，导致当连续触发 event 事件时产生问题，具体见注释。